Indirect immunofluorescent methods were used to localize [Met5]enkephalin, [Met5]enkephalin-Arg6-Phe7 and bombesin-gastrin-releasing-like immunoreactivities in human paravertebral sympathetic ganglia. Peptides were found within nerve fibers and terminals, but not cell bodies. Immunoreactivities were demonstrated to bombesin-gastrin releasing peptide and Met5-Enkephalin-Arg6-6ly7-Leu8- in small intensely fluorescent (SIF) cells and nerve fibers of rat sympathetic ganglia, suggesting an endocrine function for these cells. Aging of Fischer-344 rats was shown to be associated with an increased cell number and catecholamine histofluorescence in paraganglia cells in the retroperitoneal area and the abdomen. The number of SIF cells increased in the hypogastric ganglion with age. The changes suggest that paraganglia contribute to increased levels of circulating catecholamines in the aged rat. Consistent with these findings are observations that catecholamine storing cells in paraganglia of old rats showed structural characteristics common to adrenomedullary and paraganglionic cells of young animals. Immunohistochemistry reveals increased levels of catecholamine synthesizing enzymes, tyrosine hydroxylase, dopamine-B-hydroxylase and phenylethanolamine-N-methyltransferase, demonstrating increased catecholamine synthesizing capacity.